1. Field of the Invention
The invention concerns a helical type antenna and the method of its manufacture.
Helical type antennas have the advantage of radiating an electromagnetic wave in high quality circular polarization on a wide coverage and with a transmission lobe which may be shaped as the case may be.
These characteristics make these antennas valuable in many fields of use, particularly in ground links with orbiting satellites or in mobile/relay ground links with geosynchronous satellites.
However, this type of antenna generally has four radiating cords which have to be supplied according to adequate amplitude and phase relationships. Thus, FIG. 1 a shows the four radiating cords wound on a circular sleeve with a pitch p around a directrix of the sleeve, corresponding to an angular shift of ##EQU1## and each cord is supplied with a signal having a relative, successive, angular phase shift equal to ##EQU2## For a radiating cord A 0.degree., supplied with a null relative phase signal, marked 0.degree. in FIG. 1a, the radiating cords, successively marked A-90.degree., A-180.degree., A-270.degree., are supplied with signals of the same amplitude A but with successive phases at -90.degree., -180.degree., -270.degree..
2. Description of the Prior Art
To set up the supply of antennas of this type, various methods have been proposed until now.
According to a first method, as shown in FIG. 1c, the excitation is done, firstly, through a hybrid coupler, which divides the energy into two equal amplitude channels, phase shifted with respect to each another by 90.degree.. A double symmetrizer, housed in the shaft of the antenna, enables the passage, for each of the two channels, from the coaxial line to the diametrically opposite cords. These latter cords are therefore supplied by equal amplitudes in phase opposition. The use of a compensated symmetrizer makes it possible to adjust the operational range of frequency of the antenna.
In a second method, as shown in FIG. 1d and as in the case of FIGS. 1b and 1c, the hybrid coupler enables the energy to be separated into two equal amplitude channels in phase quadrature.
The energy is then conveyed to the supply point by two of the radiating cords which are, in fact, formed by coaxial cables. Then it gets divided, with equal amplitudes and in phase opposition, between the diametrically opposite cords, a first part being connected to the cores of the coaxial cables and another part being formed by the external part of the sheathing of the coaxial cables themselves.
As compared with the previous approach according to FIGS. 1b and 1c, this approach has the advantage of eliminating the central symmetrizer. However, its frequency characteristic curve is narrower because of the absence of any setting.
According to a third approach, as shown in FIG. 1e, the coaxial supply line is split at its end to form a symmetrizer. The distribution of the energy in quadrature between the two bi-helical elements is achieved by adjusting the length, and hence the reactance, of the radiating cords.
This approach makes it possible, advantageously, to eliminate the hybrid coupler, but it calls for the delicate setting of the length of the cords. Furthermore, since these cords have different lengths, the geometry of the antenna no longer has rotational symmetry, and the making of the antenna is more complicated.
According to a fourth approach, as shown in FIG. 1f, which is the simplest approach from the theoretical point of view, the radiating cords are supplied by a distributor.
These distributor circuits are formed by discrete elements which have to be connected to the antenna by four connections, and it is sometimes difficult to adapt this approach to the geometry of the antenna.
In all the above-mentioned cases, the other end of the cords, with respect to the end forming the supply point, is either in an open circuit, in which case the length of the cords is equal to an odd whole number of quarter wavelengths, or it is a short circuit with a length of cords equal to a whole number of half wavelengths. In practice, a true open circuit is impossible to achieve, unlike an efficient short circuit. This is why the four cords are generally short circuited together at the end opposite to the supply point, and the short circuit is made in the shape of a cross as shown in FIG. 1g.